Really there are a few more examples so don't go saying things in general because it's easier, for it is wrong and makes you look like an ass :/
I'm not going case by case, but there is currently no ADC that absolutely needs levels more than items with the only exception being the ones who need their AA modifiers and use them constantly, like Draven with his Q (which adds 85% Total AD extra damage to his autos), but even then, you're missing the point just like OP.
ADCs don't need levels in the sense that they might be level 9 and you level 18, if they have full build they will hurt you a lot because of their items. If, on the other hand, an Assassin with full build at level 9 jumps on you, who's level 18, odds are you're going to easily survive.
If an Assassin/Mage builds full tank and have a level advantage, they will still deal significant amounts of damage. Why do you think Tank Fizz and Tank Ekko were a thing? Why do you think we never saw a Tank ADC abuser (apart from post rework Graves, who's not really an ADC).
Even for drastic cases like Draven who absolutely relies on his Axes, they represent, in a full crit build (which is the highest DPS build for any champion), less than 1/3 of his damage. Comparing that to Kha'Zix, who deals a base of 806 damage by himself and it scales with 300% Bonus AD. Assuming a 400 AD build this means about 300 Bonus AD and 900 extra damage. Basicly representing 50% of his damage through items and 50% through his base damage. Much different from Draven, who has 66% coming through Items and 33% through his Q.
The less steroids an ADC has, the smaller the need for levels and the higher the need for items.
EDIT: Let's go through some maths with Kog, since you used him as an example.
His W, Bio-Arcane Barrage, deals 7% of his target's max health as Magic Damage on hit.
Assuming an average of 3500 hp for his target (which would be on a very tanky team, because with squishies running under 2000 hp, the 3500 assumption is likely much above the average), his W will be dealing 245 damage on hit.
With his natural AS scalling, he'll end up at 0,96 AS at 18. With his Q maxed, it goes to 1,197 AS.
Those two abilities combined will cause a DPS consisting of: (Base AD[98.4] + 245)*1,197
This equals to 411.04
If we give this Kog a Full build with 
, he will have the following stats:
313,4 AD
100% Crit Chance
50% extra Crit Damage
1.828 AS
And his DPS becomes ((313.4[AD] + 245[W])*2,5)*1.828) that equals to 2551.88
Even if we take away the DPS given by his base stats, W and Q, we end up with 2140.84 damage JUST FROM THE ITEMS
With items, a Kog will deal 6.208 times MORE DAMAGE than he would with just his abilities.
I don't have the capacity to make this any more cleared than this.
needs his passive to do 60% damage
needs his Q cd and scaling at its max
needs his E cd to be low to do lucian's things
needs her Q cd to be 1 second
needs her Q scaling at its maximum
needs her ulti to give 70 AD
needs his passive to do 25% missing hp damage
needs his passive % AD conversion to be at its max
needs his Q to have its scaling at its max
needs his W to be at lvl 5 to have a high duration root and be able to escape people/helping his allies
needs his passive attack speed to give a maximum of attack speed (which i think reaches the maximum at lvl 13)
needs his Q cd to be 4.5 sec to reduce his E cd and be able to poke people/kit people with IBG
needs his E cd to be low to do ezreal's things
all need lvls
They need to get a high rank on their spell to do what they are supposed to do
who needs to max W for more % damage and then Q for the extra attack speed if i recall. Jinx is similar with her Q and Ashe as well.
Really there are a few more examples so don't go saying things in general because it's easier, for it is wrong and makes you look like an ass :/
